1. Field of the Invention
The present invention pertains to a digital-analog converter having a sigma delta cascade modulator with two outputs, and more particularly to a third order sigma delta 2+1 modulator.
2. Description of the Related Art
The two output signals of a third order sigma delta modulator must be combined by means of an appropriate reconstruction transfer function to get a correct output signal.
A digital-analog converter has a sigma delta modulator with two outputs that are connected to a reconstruction circuit and then to a leveling filter. The reconstruction circuit transfer function depends on the modulator topology. In the case of a sigma delta modulator of the type 2+1, the reconstruction transfer function is the following:
Yout=Y1xe2x88x92(1xe2x88x92Zxe2x88x921)2*Y2xe2x80x83xe2x80x83(1)
where Yout is the combined signal, Y1 is the signal present at a first output, Y2 is the signal present at a second output, and Z corresponds to the Z transform.
The transfer function should be realized at a low cost, and it should not influence the system performance in terms of signal-to-noise ratio and distortion.
A reconstruction circuit is effectuated by means of digital circuits. The signal after the reconstruction has a resolution of a thermometric code with 6 bits, so that a digital-analog converter is necessary to send the signal to the leveling filter that normally is analogical. The resolution of this digital-analog converter is equal to that of the whole structure. Therefore, linearisation techniques are necessary to reach the requested distortion performances.
Another reconstruction circuit is realized by means of analogical circuits. This system is based on the fact that a leveling filter is constituted by a cascade of integrators. In fact, if the signal is applied to the input of an integrator inside the cascade, its equivalent transfer function is composed by N derivatives in cascade where N is the number of integrators between the filter main input and this additional input. Since the reconstruction function generally uses only derivatives and multiplier factors to adapt the filter coefficients, by opportunely choosing the additional input point it is possible to realize the desired transfer function. In the case of the use of the transfer function above, the signal Y1 is applied to the filter main input, the signal Y2 to the third integrator input, and the opportune multiplier factors are chosen to get the desired gain. The disadvantages are in the fact that in order to reach the desired reconstruction function structure, limitations are necessary, and the performances depend on the capacitor tolerances and on the parameters of the operational amplifiers used as integrators, like the finished gain values, the speed and the rise time.
The disclosed embodiments of the present invention are directed to a digital-analog converter having a sigma delta modulator of the type with two outputs and a reconstruction circuit that does not require a digital-analog converter, is insensitive to the operational amplifier parameters, and does not impose limits in the choice of the leveling filter.
In accordance with the present invention, the foregoing and other objects are attained by means of a digital-analog converter having a sigma delta modulator of the type with two outputs able to provide a first and a second signal at said two outputs; a reconstruction circuit of said first and second signals able to provide a reconstructed signal; a filter able to filter said reconstructed signal; the reconstruction circuit configured to combine said first and second signals according to the following relationship:
xe2x80x83Yout=Y1*(1+Zxe2x88x921)xe2x88x92Y2*(1Zxe2x88x921)+Y2*Zxe2x88x922*(1xe2x88x92Zxe2x88x921),
where:
Yout corresponds to said reconstructed signal,
Y1 corresponds to said first signal,
Y2 corresponds to said second signal,
Z corresponds to the Z transform.
In accordance with another embodiment of the invention, a digital-analog converter is provided that includes a sigma delta modulator of the type having a first and a second output able to provide a first and a second output signal; an operational amplifier having a positive input and a negative input, said negative input is connected to a first node; a first switch connected between said first node and a second node; a second switch connected between said first node and a third node; a third switch connected between said first node and a fourth node; a fourth switch connected between said second node and a first prefixed bias voltage; a fifth switch connected between said third node and a second prefixed bias voltage; a sixth switch connected between said fourth node and a third prefixed bias voltage; a first capacitor connected between said second node and a fifth node; a second capacitor connected between said third node and a sixth node; a third capacitor connected between said fourth node and a seventh node; a seventh switch connected between said fifth node and a first input; an eighth switch connected between said sixth node and a second input; a ninth switch connected between said seventh node and a third input; a tenth switch connected between said fifth node and a fourth input; an eleventh switch connected between said sixth node and a fifth input; a twelfth switch connected between said seventh node and a sixth input; said first input is applied to said first output; said second input is applied to said first inverted output; said third and said fourth input are applied to said second delayed twice and inverted output; said fifth and said sixth input are applied to said second output.
Because of the present invention, it is possible to realize a low cost analogical type reconstruction circuit, simple and with good performance in terms of signal-to-noise ratio and distortion.